I don't understand why they're filtering base? Any ideas?

The <base> tag specifies a default address or a default target for all links on a page. Therefore, using that tag, people could put in an off site URL in a tag other than an a href link (the other thing that was changed, by necessitating quotes). Both things are part of the link security they just changed. Apparently they are filtering any 'base' that is inside of <>s for the security right now, hope the current filter is a stop gap then while they tweak it...